Google webmaster tools is helpful. You'll see errors when google bot tries to crawl a page that doesn't exist (for example, because you deleted it and there is

Google webmaster tools is helpful. You'll see errors when google bot tries to crawl a page that doesn't exist (for example, because you deleted it and there is a link to it somewhere). Keep an eye on it, and it will help point out problems. It also provides a partial list of incoming links to start with - as good or better than any other free tool I'm aware of.